If you want to understand the essential change that has engulfed the British military in modern times, consider this. During Sir Mike Jackson's time as a soldier - he retired as head of the Army in 2006 after 46 years in the forces - the size of the Army had halved from approximately 200,000 to roughly 100,000. During that same time, however, the size of the Army legal service doubled.

The way General Jackson told it at Hay on Monday evening in a conversation with Nik Gowing, the essentials of soldiering remain fundamentally unaltered in the 21st century. (I say a conversation with Nik Gowing, because this is how the event was billed, but this is misleading; the session could more accurately be labelled Nik Gowing talks at length about military matters and the career of Sir Mike Jackson, with occasional comments from the general when he was permitted to get a word in).

But back to the real theme. General Jackson reckons that the military basics are as they always were. The Army has to work hard to recruit and retain, but today's soldiers are just as keen as their forebears - and in similar ways. They are brave. They are trained. They are professional. They are equipped. They are disciplined. They long to go on real operations, the summit of their soldiering. When they go into battle or conflict, they go "with courage and commitment", just as their fathers and grandfathers did (this ringing assertion drew warm applause).

Yet not even this affirmation - true as it may be in many ways - can disguise the ways in which things today are very different, not just (though it's obviously very important) because of divisions and bitterness over Iraq. Soldiers may want to fight, today as in the past. But the scrutiny under which they do so, the relative transparency of what they do compared with even the recent past, and the accountability to which they and their commanders are now held, is of altogether another order.

In his book Soldier, Jackson discusses several of these problems at length. Because the professional discretion of a commanding officer is second nature to him, his account is occasionally blunted rather than blunt. But it is clear from the book that these issues of scrutiny, transparency and accountability are ever-present problems for the modern commander, not some afterthought when the dust of battle settles. We are all subject to the law at all times, says Jackson. It is the difference, he said at Hay, between having an army and having a rabble. Yet the reality is that lawfare is gradually mounting a challenge to warfare as any general's prime concern.

Part of this is about the conditions in which soldiers serve. But it is also about a change in the culture. If barracks or family quarters are a slum, soldiers won't merely not serve, they'll sue. If their kit doesn't work, doesn't protect them, or is manifestly too old, they won't simply get wounded or die, their families will campaign about it, newspapers will take it up and coroners and judges will feel entitled to weigh in. In a society in which human rights lawyers feel not merely empowered to challenge decisions at every level, but in which they are also actively seeking out opportunities to do so, the age of do or die is clearly dead.

Whether this is a good thing or a bad thing is an interesting question but not the main one. The point is that it is happening. As even General Jackson conceded, soldiers now go into action within "a more demanding and complex legal environment" than they once did. This can, he said, be irritating and frustrating at times, but it is the bedrock upon which soldiering must now be carried out. There was no place in the Army, he added, for abuses of human rights, whether of or by service personnel. The business of soldiering will not change, he assured us.

All this is euphemism and understatement. The reality is that things have changed, and probably changed for ever. Law-law, rather than jaw-jaw, is the context of today's war-war. The fog of war no longer protects the deeds of war. People feel entitled to know what happens. And even General Jackson's book, and his presence at Hay to promote it, are part of this same inexorable and subversive process.